As used herein, the term "fiber" includes fibers of extreme or indefinite length (filaments) and fibers of short length (staple).
As used herein, the term "solution-dyed" describes an item that has been colored by the introduction of colorants into the polymer melt or spinning solution prior to shaping.
As used herein, the term "colorant" means a coloring agent which is a water-soluble dye, organic-soluble dye, polymer-soluble dye, pigment or any color imparting agent.
Precise measurement and control of the concentration of color in polymer processing is essential for ensuring that the end products, for example, yarn made into fabric or carpet, is the desired color. Consistency in color is important in the manufacture of textiles and carpets to allow color matching of products made from different lots of materials.
Traditionally, the concentration of colorants in a stream of molten polymer has been measured off-line by forming fibers or plaques and then taking measurements from these samples. There are several drawbacks to these off-line procedures. First, the procedure is time-consuming. The concentration of color is adjusted into the appropriate value by trial and error. Color values are not readily available. Also, the time to obtain results is usually hours.
Several in-line methods for measuring various attributes of polymer melt flows have been proposed. These methods, however, also suffer from various drawbacks. For example, U.S. Pat. No. 4,516,864 to Kim et al. ("Kim") teaches the use of infrared radiation directed through a fiber optic cable to effect temperature measurements. These measurements are accomplished by placing windows opposite each other in the flow channel. The use of two windows requires invasion of the flow in two places with diametrically opposing ports.
Another example of melt stream analysis is found in U.S. Pat. No. 4,529,306 to Kilham et al. ("Kilham '306"). Kilham '306 relies on the fact that polymers are virtually transparent when molten and passes a beam of visible light through a flow of the polymer melt to observe the illuminated polymer stream either visually or using a video camera. The system employs fiber optics to transmit the illumination and allow the observation of the illuminated stream. The Kilham '306 invention relates to the detection of impurities and gels within a polymer melt and no quantitative analysis is performed. Furthermore, Kilham '306 requires two points of intrusion into the molten polymer path, one for providing visible light and the other for observing the illuminated flow.
U.S. Pat. No. 4,910,403 to Kilham et al. ("Kilham '403") teaches the use of a specially designed diamond flow cell to effect measurements using near to far infrared energy. The sample cell possesses unique characteristics with regard to temperature and pressure resistance. A beam of energy is passed through the sample so that the method involves two invasion points. The use of this flow cell requires the melt flow equipment to be specially adapted.
U.S. Pat. No. 5,062,713 to Farquharson et al. ("Farquharson") discloses a method for determining the residence time distribution of a polymer extruder. This process is useful in designing extruders for optimum performance. The invention passes energy in the visible region through the polymer melt to determine the relative concentration of a colorant in the melt. The colorant is added as a tracer or marker and used to measure the residence time of the colored portion of the polymer. Only two wavelengths are supplied and measured. One wavelength is used to quantify the colorant and the other is used to identify the background spectrum. As with the others, the Farquharson invention requires two points of invasion into the polymer flow system. One of the points is for providing a light source and diametrically opposed to the light source is a detector for detecting transmitted light energy. Farquharson requires also that an optimum wavelength is pre-selected for use.
Fiber optics have been used inline to measure specified properties in other fields, too. See, for example, Stephen J. Swarin and Charlene A. Drumm, "Predicting Gasoline Properties Using Near-IR Spectroscopy", Spectroscopy, Sep. 1992, pp. 42-49.
The present invention provides a number of advantages over the discussed art. First, only one window into the process is needed. Also, visible light is used to characterize the color of the polymer melt. Further, the characterization of the color is accomplished by using a plurality of wavelengths covering the entire visible spectrum, eliminating the need to optimize and pre-select a wavelength. Meaningful color measurements are obtained in minutes, not hours.